Access to Naver, South Korea's largest Internet portal, is completely blocked in China.
Internet users in major Chinese cities, including Beijing and Shanghai, have been complaining since Friday afternoon that they cannot access the site or any of its services.
Naver Blog and its community service Naver Cafe have been cut off since October.
Daum, another South Korean portal, has also been blocked in China since January.
An expert speculates the Beijing government may have completely blocked Naver through an Internet service provider.
Another industry official said it appears China is exerting control on service providers because if the government directly blocks access, it needs to issue a notice.
As for possible reasons for the block, the official cited the 30th anniversary of the Tiananmen Massacre earlier this month and the eruption of mass protests in Hong Kong against a controversial extradition bill.